In recent years, for the purpose of enhancement of productivity in reading an document, protection of the document, improvement in quietness, and/or the like, image readers having a simultaneous duplex scanning mechanism capable of simultaneously reading both sides of the document have been proposed. In such image forming apparatuses, in general, to read the front side of the document, a charge coupled device (CCD) is used similarly to conventional image forming apparatuses and, to read the back side of the document, a contact image sensor (CIS) is used (for example, see Japanese Patent Application Laid-open No. 2009-33723 and Japanese Patent Application Laid-open No. 2005-217561).
Some CISes have an image processing function within a CIS module. With such CISes, while it is necessary to carry out inter-line correction in the CIS module before image processing, some may assume that the scanning is carried out at a fixed resolution (scan speed) in a sub-scanning direction to cut cost by reducing the memory for the inter-line correction.
With such a fixed resolution, the CIS cannot carry out magnification mechanically by varying the resolution even if the magnification is specified. Accordingly, even when the magnification is specified, the CIS consistently scans the document at the fixed resolution (actual size magnification) regardless of a magnification ratio. An image processing section or a control section at a subsequent stage then magnifies the image data read by the CIS in an electronic magnification process at the magnification ratio specified.
For example, it is assumed that the fixed resolution is 600 dots per inch (dpi) and a magnification of 50% is carried out. In this case, if the resolution is variable, scanning the document at 300 dpi allows the mechanical magnification to be carried out by reading the document in the sub-scanning direction at twice the speed of scanning the document at 600 dpi. However, with the CIS the resolution of which is fixed, it is not possible to carry out the mechanical magnification by scanning the document at 300 dpi and thus, the magnification at 300 dpi needs to be carried out by the electronic magnification process after the document is scanned at 600 dpi.
As described above, while magnification can be carried out by the electronic magnification process even when the resolution is fixed, compared with the mechanical magnification, the reading speed of the document is slowed and thus, the productivity is lowered.